Defensins are small, cationic peptides containing six conserved cysteine residues that form three disulfide bonds. Functional, mature defensins arise by the sequential post-translational processing of prepro-proteins that are 93-95 amino acids in length. Mature alpha-defensins generally contain 29-33 residues, whereas mature beta-defensins are more basic and are generally between 34-37 amino acids in length. The recently identified theta defensins are formed by the head-to-tail ligation of two alpha defensin-related nonapeptides, generating a circular 18-residue polypeptide.
Defensins were first identified in neutrophils and have been detected in human, rabbit, guinea pig, and rat phagocytes. Four human alpha defensins have been isolated from neutrophils: human neutrophil peptide (HNP)-1, HNP-2, HNP-3, and HNP-4. HNP-1, HNP-2, and HNP-3 are stored in the azurophilic granules of neutrophils and constitute approximately 99% of the defensin content of the neutrophils. HNP-4 is also present in these granules, but at a concentration that is equivalent to only one percent of the other HNP polypeptides. Alpha defensins also include two human enteric defensins, human defensin (HD)-5 and HD-6, which are highly expressed in epithelial cells of the small intestine, specifically in the Paneth cells.
Defensins play important roles in the innate immune defense in vertebrates. Defensins are broad-spectrum antimicrobial molecules that are released from azurophilic granules into a phagosome for the nonoxidative killing of phagocytized infectious agents such as Gram-negative bacteria, Gram-positive bacteria, fungi, and certain enveloped viruses, by forming pores in their membranes. Constitutively expressed defensins contribute to an antimicrobial barrier at the epithelial cell surface and inducible epithelial defensins are highly expressed at areas of inflammation or infection. Thus, defensins play an important role in the body's natural immunity against infections. Defensins also play a role in the body's natural immunity against tumor cells.
The ubiquitous use of antibiotics has resulted in the selection of bacteria that are relatively resistant to these drugs. Furthermore, few drugs are effective against viral and fungal microorganisms. Thus, there is a continuing need to identify novel agents that reduce or inhibit the growth of such microorganisms as well as novel agents that may recruit inflammatory cells to enhance an immune response. Defensins exhibit a wide range of antimicrobial activities, including cytotoxicity towards bacterial cells, however these proteins are also cytotoxic for mammalian cells, including human epithelial and endothelial cells. This side effect may limit their usefulness as antimicrobial agents. Thus, there also exists a need to identify new methods of modifying existing agents, such as defensins, in order to modify their cytotoxic activity and give them superior antimicrobial activities.